Organic electroluminescence device and organic electroluminescence display

ABSTRACT

An organic electroluminescence device is disclosed. The device includes an anode, a cathode formed above the anode, a light-emitting layer containing an organic light-emitting material that is formed between the anode and the cathode, a hole-injecting layer on the anode that includes a hole-transporting material and an acceptor, and an electron-transporting suppressing stack between the hole-injecting layer and the light-emitting layer. The electron-transporting suppressing stack consists of multiple carrier-transporting layers, and forms energy barriers relative to electrons flowing from the light-emitting layer to the hole-injecting layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a U.S. continuation application filed under 35 USC111(a) claiming benefit under 35 USC 120 and 365(c) of PCT applicationJP2003/004225, filed Apr. 2, 2003. The foregoing applications are herebyincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a technology for aphotoelectronic device and a flat-panel display using a photoelectronicdevice, and particularly relates to an organic electroluminescencedevice and an organic electroluminescence display.

2. Description of the Related Art

In recent years, there has been a gradual shift in market needs from aconventional large and weighty CRT (a Braun tube) display to a thin andlight-weight flat display. As for flat displays, LCD displays and plasmadisplays have been brought to commercial use as household televisionsets and PC monitors, etc.

Recently and continuing, attention is being given to anelectroluminescence display (below called “EL display”) and, morespecifically, an organic EL display, as a next-generation flat display.Since a report on a stacking device with hole-transporting andelectron-transporting organic thin-films (C. W Tang and S. A. Van Slyke,Applied Physics Letters vol. 51, 913 (1987)), an organic EL device forthe organic EL display has attracted attention as a large-arealight-emitting device for emitting light at a low voltage, not more than10 volts, and is being studied actively.

A stacking organic EL device basically has a configuration of an anode,a hole-transporting layer, a light-emitting layer, anelectron-transporting layer, and a cathode. Of these, theelectron-transporting layer, as in the case of the double-layer deviceof Tang and Van Slyke as described above, is configured such that thelight-emitting layer also serves the function of theelectron-transporting layer. For the anode, electrode materials having alarge work function such as gold (Au), tin oxide (SnO₂), and Indium TinOxide (ITO) are being used. Moreover, for the cathode, metals Li and Mghaving a low work function with a small barrier for injecting theelectrons into the electron-transporting layer, or their alloys Al—Liand Mg—Ag, etc., are being used.

Using various organic EL device structures and organic materials up tonow has produced a luminance of about 1,000 cd/m² at a light-emittingvoltage of 10 volts in the early stage of use. However, continuouslydriving the organic EL device over time results in a decreasedlight-emitting luminance and an increased drive voltage, eventuallycausing a short circuit.

It is considered that degradations of the organic EL device are due tocrystallization over time of organic materials, the associatedaccumulation of space charges within the organic layer, and polarizationdue to applying the electric field in a certain direction, causingorganic molecules to polarize the electrodes so as to change theelectric characteristic of the device, or degradations due tooxidization of the electrodes, etc. Moreover, when there is high powerconsumption, it is possible that the energy lost that changes to heathelps to degrade the organic material. Therefore, to increase the lifeof the device, desirably a highly-efficient device from which a highlight-emitting luminance can be obtained at as low a current and voltageas possible should be implemented.

Thus, to achieve the high efficiency as described above, attempts arebeing made to increase the durability with a study in terms of materialsand a method of driving the EL device. For example, as disclosed inJP06-036877A, a method is proposed that alternately repeats stacking oftwo types of organic layers for forming light-emitting layers such as tohave energy bands of well-type potentials, so that electrons and holesnot rebonding in one light-emitting layer rebond in the nextlight-emitting layer to emit light, thus increasing the light-emittingefficiency. However, with this configuration, a decreased voltage ateach organic layer and generated Joule heat due to the high-resistancecharacteristics of the organic layers lead to a decrease in thelight-emitting efficiency and the service life of the organic EL device.

In order to resolve this problem, as disclosed in JP04-297076A, aproposal is made for doping with acceptors within the hole-transportinglayer to enhance conductivity.

In this case, while it is possible that the conductivity can be enhancedto increase the amount of hole current and the amount of electroncurrent, as the carriers are not enclosed sufficiently, a problem arisessuch that the power consumption increases and the light-emittingefficiency and the service life decrease. Here, it is considered that asthe electron affinity of the acceptors is generally greater than that ofhole-injecting-transporting materials, decreasing the energy barrier atthe interface of the hole-transporting layer and the light-emittinglayer, which energy barrier encloses electrons within the light-emittinglayer, makes efficient enclosing of electrons within the light-emittinglayer not possible, causing the light-emitting efficiency to decrease.

As means for resolving this problem, as disclosed in JP2000-196140, amethod is proposed for forming an electron-injecting suppressing layerthat encloses electrons between the light-emitting layer and thehole-transporting layer, thus increasing the light-emitting efficiency.While there is less decrease in the light-emitting efficiency for a caseof forming the electron-injecting suppressing layer than for a case ofthe hole-transporting layer directly bordering on the light-emittinglayer, there is a problem that electrons that can pass through theelectron-injecting-suppressing layer exist. While it is possible toincrease the thickness of the electron-injecting suppressing layer so asto suppress such electrons as described above, at the same time, theflow of holes is suppressed, causing the luminance to decrease, so thatthere is yet to be a solution having satisfactory EL characteristics.

Patent Document 1

JP2000-196140A

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a technologyfor a photoelectronic device and a flat-panel display using aphotoelectronic device that substantially obviates one or more problemscaused by the limitations and disadvantages of the related art.

It is a more particular object of the present invention to provide anorganic electroluminescence device and an organic electroluminescencedisplay that have superior light-emitting efficiency and a long servicelife.

According to the invention, an organic electroluminescence deviceincludes an anode, a cathode formed above the anode, a light-emittinglayer containing an organic light-emitting material that is formedbetween the anode and the cathode, a hole-injecting layer on the anodethat contains a hole-transporting material and an acceptor, and anelectron-transporting suppressing stack between the hole-injecting layerand the light-emitting layer, wherein the electron-transportingsuppressing stack consists of multiple carrier-transporting layers, andforms energy barriers relative to electrons flowing from thelight-emitting layer to the hole-injecting layer.

The organic electroluminescence device as described above has superiorlight-emitting efficiency and a long service life.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an organic EL device according to afirst embodiment of the present invention;

FIG. 2 is an energy diagram of the organic EL device according to thefirst embodiment;

FIG. 3 is a cross-sectional view of the organic EL device according to avariation of the first embodiment of the present invention;

FIG. 4 is a graph for describing a method of deriving energy gaps;

FIG. 5 is a graph for describing a method of deriving ionizingpotentials;

FIG. 6 is a table illustrating the characteristic values of theelectron-transporting suppressing layer, the hole-injecting layer, andthe light-emitting layer used for the organic EL devices according toexamples according to the present invention and comparative examples;

FIG. 7 is a table illustrating the layer configuration of specificelements of organic EL devices according to a first example, and firstthrough third comparative examples; and

FIG. 8 is an exploded perspective view of the organic EL displayaccording to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments of organic EL devices according to thepresent invention are described with reference to the accompanyingdrawings.

A First Embodiment

FIG. 1 is a cross-sectional view of an organic EL device according tothe first embodiment of the present invention. FIG. 2 is an energydiagram of the organic EL device according to the first embodiment asillustrated in FIG. 1. In FIG. 2, Ea denotes the electron affinity andIp denotes the ionizing potential. Referring to FIGS. 1 and 2, anorganic EL device 10 is configured with a substrate 11, an anode 12, ahole-injecting layer 13, an electron-transporting suppressing stack 14,a light-emitting layer 15, an electron-transporting layer 16 and acathode 18 that are sequentially formed on the substrate 11.

The organic EL device 10 of the present invention has the hole-injectinglayer 13 composed of a hole-transporting material that is doped withacceptors. The hole-injecting layer 13 is, for example2-TNATA(4,4′,4″-tris(2-naphthylphenylanimo)triphenylamine), ahole-transporting material doped with inorganic materials such ashalogen metal compounds, halogens, and platinum-group element metals, ororganic materials having a cyano group or a nitro group as acceptors.

More specifically, as a hole-transporting material a known material maybe used. Moreover, as preferable acceptors, the halogen metal compoundsinclude FeCl₃, InCl₃, AsF₆, etc., the halogens include Cl, Br, I, etc.,and the platinum-group element metals include Au, Pt, W, Ir, etc.Furthermore, as preferable acceptors, the organic materials having acyano group include TCNQ(7,7,8,8,tetracyanoquinonedi-methane),F4-TCNQ(2,3,5,6-tetrafluoro-7,7,8,8,tetracyanoquinonedimethane),TCNE(tetracyanoethylene), etc., and the organic materials having a nitrogroup include TNF(trinitrofluorenone) and DNF(dinitrofluorenone), etc.Of these, TCNQ, F4-TCNQ, TCNE, TNF, and DNF are especially preferable.

The content of the acceptors in the hole-injecting layer 13 is set at0.01 vol. % to 2.0 vol. %, preferably at 0.05 vol. % to 1.0 vol. %,relative to hole-transporting materials (100 vol. %). Below 0.01 vol. %,the conductivity does not increase, while above 2.0 vol. %, the amountof current increases, causing the power consumption to increase. Morespecifically, when using F4-TCNQ as the acceptors, the content of theacceptors is preferably at 0.05 vol. % to 1.0 vol. %.

The hole-injecting layer 13 may be formed using such vacuum processes asvacuum deposition, CVD and sputtering, or such wet processes as spincoating and printing.

The organic EL device of the present invention is provided with theelectron-transporting suppressing stack 14 between the hole-injectinglayer 13 and the light-emitting layer 15. The electron-transportingsuppressing stack 14 is formed sequentially stacked from thelight-emitting layer 15 side toward the hole-injecting layer 13, by afirst electron-transporting suppressing layer 14A₁, a secondelectron-transporting suppressing layer 14B₁, and a firstelectron-transporting suppressing layer 14A₂. The firstelectron-transporting suppressing layers 14A₁ and 14A₂, and the secondelectron-transporting suppressing layer 14B₁ that are composed fromhole-transporting materials are arranged such that energy barriers areformed relative to the electrons from the light-emitting layer 15 sideto the hole-injecting layer 13 side.

The first electron-transporting suppressing layers 14A₁ and 14A₂, andthe second electron-transporting suppressing layer 14B₁ preferably havea relationship as represented in Equation (1) below:E_(aHT1)>E_(aHT2)   (1)Here, E_(aHT1) is the electron affinity of the firstelectron-transporting suppressing layers 14A₁ and 14A₂, while E_(aHT2)is the electron affinity of the second electron-transporting suppressinglayer 14B₁. As the second electron-transporting suppressing layer 14B₁for which the amount of the electron affinity is larger relative to thefirst electron-transporting suppressing layers 14A₁ and 14A₂ is formedbetween the two first electron-transporting suppressing layers 14A₁ and14A₂, an energy barrier BR₁ is formed relative to the electrons at theinterface from the first electron-transporting suppressing layers 14A₁to the second electron-transporting suppressing layer 14B₁. Thus, theamount of electron current from the light-emitting layer 15 to thehole-injecting layer 13 is reduced.

Moreover, in addition, the light-emitting layer 15 and the firstelectron-transporting suppressing layer 14A₁ preferably have arelationship as represented in Equation (2) below:E_(aHT1)<E_(aEM)   (2)Here, E_(aEM) is the electron affinity of the light-emitting layer 15,while E_(aHT1) is the electron affinity of the firstelectron-transporting suppressing layer 14A₁. Also an energy barrier BR₂is formed at the interface between the light-emitting layer 15 and thefirst electron-transporting suppressing layer 14A₁ so that the electronsare enclosed in the light-emitting layer 15. It is noted that theprobability of electrons crossing the energy barrier BR₂ is high withthe first electron-transporting suppressing layer 14A₁ being only onelayer, and increasing the thickness of the first electron-transportingsuppressing layer 14A₁ in order to reduce such probability as describedabove causes a large voltage drop that is not desirable in terms ofdurability. Moreover, such measure as described above may cause adecrease in the amount of hole-electron current. Thus, providing at theanode side of the light-emitting layer 15 the electron-transportingsuppressing stack 14, which consists of multiple first and secondelectron-transporting suppressing layers and which has multiple energybarriers BR₁ and BR₂, effectively suppresses the amount of electroncurrent proceeding from the light-emitting layer 15 toward thehole-injecting layer 13 and increases the space-electron density in thelight-emitting layer 15 so as to increase the light-emitting efficiency.

Below, a specific configuration of the organic EL device according tothe present embodiment is described.

For the substrate 11, what may be used are, for instance, suchtransparent base materials as glass, quartz, etc., such semiconductorsubstrates as Si, etc., such films as PET and PEN, etc., and such resinsubstrates as PVA. Or TFTs (Thin-Film Transistors) for controlling onand off of the organic EL device may be formed in a matrix shape onthese substrates. The thickness of the substrate 11 that is selectedaccordingly depending on the materials of the substrate 11 isapproximately 0.1 mm to 10 mm.

The anode 12 is formed on the substrate 11 by deposition or sputteringfrom conducting materials such as Al, and preferably such materials asAu, Cr, etc., which have a large work function are used from thehole-injection performance point of view. It is noted that when light isirradiated from the anode side, the anode 12 is formed from transparentmaterials such as ITO and IZO (Indium-Zinc-Oxide).

As hole-transporting materials for the hole-injecting layer 13, thefirst electron-transporting suppressing layer 14A, and the secondelectron-transporting suppressing layer 14B, materials having a highHOMO, or a small ionizing potential are used. Representative materialsinclude copper phthalocyanin (CuPc), starburst amine m-MTDATA, 2-TNATA,TPD, α-NPD, etc.

It is noted that the ionizing potentials of the firstelectron-transporting suppressing layer 14A and the secondelectron-transporting suppressing layer 14B preferably are roughly equalfrom the point of view of increasing the amount of hole current. Theenergy barriers may be formed low relative to the holes, suppressing theamount of electron current without inhibiting the flow of hole current.

Moreover, a further non-acceptor containing hole-injecting layer may beprovided between the hole-injecting layer 13 and the anode 12. Thefurther hole-injecting layer which is to made to be a thin layerrelative to the hole-injecting layer 13 is composed from a materialhaving an ionizing potential that is almost equal to the size of thework function of the anode 12. Such measure as described abovefacilitates the flow of hole current.

For the light-emitting layer 15, such metal complex-based materials asAlq3(tris(8-hydroxyquinolio)aluminum), Znq2, Balq2, and suchpigment-based materials such as PZ10 and EM2 may be used. Moreover, hostmaterials such as Alq3 that are doped with such pigments as Levulan andTPB may be used.

For the electron-transporting layer 16, 8-hydrooxyquinoline metalchelates, metal thioxynoid compounds, oxadiazole metal chelates,triazine, 4,4′-bis(2,2-diphenyl vinyl)biphenyl, etc., may be used.Preferred 8-hydrooxyquinoline metal chelates include Alq3,Balq(bis(8-hydroxynolate)-(4-phenylphenolate)aluminum), bis-PBD, etc.Moreover, preferred metal thioxynoid compounds includebis(8-quinolinethiolate)zinc, bis(8-quinolinethiolate)cadmium,tris(8-quinolinethiolate)gallium, tris(8-quinolinethiolate indium), etc.Furthermore, preferred oxadiazole metal chelates includebis(2-(2-hydroxylphenyl)-5-phenyl-1,3,4-oxyadiazolate)zinc,bis-(2-(2-hydroxylphenyl)-5-phenyl-1,3,4-oxyadiazolate)beryllium,bis(2-(2-hydroxylphenyl)-5-(1-naphthyl)-1,3,4-oxyadiazolate)zinc,bis(2-(2-hydroxylphenyl)-5-(1-naphthyl)-1,3,4-oxyadiazolate)beryllium,etc.

For the cathode 18, metals such as Li and their alloys Mg—Ag, Al—Li,etc., that have a small work function are used. Moreover, a cathode thatintroduces an electron-injecting layer of metal fluorides, etc., such asLiF/Al may be used.

As illustrated in FIG. 2, the electrons tend to flow from thelight-emitting layer 15 towards the hole-injecting layer 13. However,the energy barrier BR₁ is formed at the interface between the firstelectron-transporting layer 14A₁ and the second electron-suppressinglayer 14B₁ due to the difference in the electron affinities of these twolayers E_(aHT1)−E_(aHT2). Hereby the flow of electrons is prevented andthe amount of electron current is suppressed, enclosing the electrons inthe light-emitting layer 15. Moreover, the energy barrier BR₂ is formedat the interface between the light-emitting layer 15 and the firstelectron-trasnporting layer 14A₁ due to the difference in the electronaffinities of these two layers E_(aEM)−E_(aHT1). Hereby furthermore theelectrons are enclosed in the light-emitting layer 15. Thus thespace-electron density of the light-emitting layer 15 is increased,increasing the probability of rebonding with the holes.

In the organic EL device of the present embodiment, the ionizingpotentials of the electron-transporting layer 16 bordering the cathode18 side of the light-emitting layer 15 and of the light-emitting layer15 preferably have a relationship as represented in Equation (4) below:I_(pEM)<I_(pET)   (4)

Here, in the Equation (4), I_(pEM) is the ionizing potential of thelight-emitting layer 15, I_(pET) is the ionizing potential of theelectron transporting layer 16 as described above, the ionizingpotentials being represented as the difference (a positive value)between the valence electron level (the energy at the upper end of thevalence-electron band) and the vacuum level. Providing an energy barrierBR₃ at the cathode 18 side of the light-emitting layer 15 encloses theholes in the light-emitting layer 15. Such measure as described aboveincreases the space-hole density, increasing the probability ofrebonding with electrons.

FIG. 3 is a cross-sectional view of the organic EL device according to avariation of the present embodiment. In the figure, those partscorresponding to the parts described previously are given the samereference letters so as to abbreviate the descriptions.

Referring to FIG. 3, an organic EL device 20 of the present variation isconfigured with a substrate 11, and an anode 12, a hole-injecting layer13, an electron-transporting suppressing stack 24, a light-emittinglayer 15, an electron-transporting layer 16, and a cathode 18 that aresequentially formed on the substrate 11. The electron-transportingsuppressing stack 24 is composed from a first set of a firstelectron-transporting suppressing layer 24A₁ and a secondelectron-transporting suppressing layer 24B₁ to an N-th set of the firstelectron-transporting suppressing layer 24A_(N) and the secondelectron-transporting suppressing layer 24B_(N). Here, the number ofsets to be stacked N is an integer no less than 2, and the N-th set maybe formed with only the first electron-transporting suppressing layer24A_(N).

For a larger number of sets to be stacked N, the firstelectron-transporting suppressing layer 24A and the secondelectron-transporting suppressing layer 24B are set to be thin. Thetotal thickness of the electron-transporting suppressing stack 24preferably is set at 150 nm to 500 nm. Setting the total thickness asdescribed above 500 nm results in a large electric resistance, making itnot possible to put through the current sufficiently, and applying ahigh voltage causes the service life of the organic EL device 20 to bereduced. Furthermore, setting the total thickness as described below 150nm makes it not possible to sufficiently suppress the amount of electroncurrent flowing from the light-emitting layer 15 side to thehole-injecting layer 13.

More specifically, the thickness of each of the firstelectron-transporting suppressing layers 24A and the secondelectron-transporting suppressing layers 24B is set at 2 nm to 50 nm,and more preferably at 2 nm to 20 nm. Making the firstelectron-transporting suppressing layers 24A and the secondelectron-transporting suppressing layers 24B thin encloses the electronsin the light-emitting layer 15 and prevents blocking the flow of holes.

According to the present variation, providing the electron-transportingsuppressing stack 24 with a large number of sets each consisting of afirst electron-transporting-suppressing layer 24A and a secondelectron-transporting suppressing layer 24B and thus with a large numberof energy barriers relative to the electrons suppresses more reliablythe flow of electrons and prevents blocking the flow of holes.

It is noted that the electron-transporting suppressing stacks 14 and 15in the organic EL devices 10 and 20 according to the first embodimentand the variation as described above, are composed of two types ofelectron-transporting suppressing layers, but may be composed of threeor more types of electron-transporting suppressing layers. For example,using a third electron-transporting suppressing layer as well as thefirst and second electron-transporting suppressing layers, from thelight-emitting layer 15 side to the hole-injecting layer 13, the layersare set in the order of the first electron-transporting suppressinglayer/the second electron-transporting suppressing layer/the firstelectron-transporting suppressing layer/the second electron-transportingsuppressing layer/the third electron-transporting suppressing layer/thefirst electron-transporting suppressing layer. The relationship amongthe electron affinities of the three types of the electron-transportingsuppressing layers is set as represented in Equation (5) below:E_(aHT1)>E_(aHT2)>E_(aHT3)   (5)

Here, in the Equation (5), E_(aHT1) is the electron affinity of thefirst electron-transporting suppressing layer, E_(aHT2) is the electronaffinity of the second electron-transporting suppressing layer, andE_(aHT3) is the electron affinity of the third electron-transportingsuppressing layer. Forming relative to the electrons an energy barrierat the interface of the second electron-transporting suppressing layerand the third electron-transporting suppressing as well as an energybarrier at the interface of the first electron-transporting suppressinglayer and the second electron-transporting suppressing layer, thusforming within the electron-transporting suppressing stack energybarriers having different sizes, enhances the suppressibility of theflow of electrons.

Moreover, the first electron-transporting suppressing layer 14A or thesecond electron-transporting suppressing layer 14B bordering the cathode18 side of the hole-injecting layer 13 preferably has a relationship asrepresented in Equation (3) below:E_(aHT)>E_(aHI)   (3)

Here, in Equation (3), E_(aHT) is the electron affinity of the first orthe second carrier transporting layer bordering the cathode 18 side ofthe hole-injecting layer 13, and E_(aHI) is the electron affinity of thehole-injecting layer 13. Providing an energy barrier at a location priorto the electrons reaching the hole-injecting layer 13 when flowing fromthe light-emitting layer 15 side to the hole-injecting layer 13suppresses the flow of electrons.

It is noted that in the present embodiment, the energy gap, the ionizingpotential, and the electron affinity of each of theelectron-transporting suppressing layers 14A, 14B, 24A, 24B, thelight-emitting layer 15, and the electron-transporting layer 16, etc.,were derived according to those measurement conditions and measurementmethods as described below.

As for the energy gap Eg, the light-absorption spectrum was measured,setting the energy at the long-wavelength end of the light-absorptionspectrum as the energy gap Eg. More specifically, under the sameconditions as the conditions for forming each layer of the organic ELdevices as described above, a particular one of theelectron-transporting layers, etc., to be measured was individuallyformed at a thin film having a thickness of 50 nm. Using aspectrophotometric apparatus (manufactured by Hitachi, Co. Ltd., productname: Spectrophotometer-U-4100), ultra-violet to visible light wasirradiated on a thick film in the atmosphere and the light-absorptionspectrum (the wavelength dependency) was measured.

FIG. 4 is a characteristic diagram illustrating the light-absorptionspectrum. Referring to FIG. 4, a point of intersection CP1 of a straightline extrapolated to the long-wavelength side using a straight-lineapproximation of a straight-line segment LN1 at the foot of thelong-wavelength side of the light-absorption spectrum, and a straightline extrapolated to the short-wavelength side using a straight-lineapproximation of a straight-line segment BG1 at the background isconverted to the energy gap Eg.

As for the ionizing potential Ip, the photoelectron-emission energythreshold measured using an ultra-violet photoelectron spectroscopy wasset to be the ionizing potential Ip. More specifically, using a thickfilm formed in the same manner as the thick film used for measuring theenergy gap Eg, with an ultra-violet photoelectron spectroscopicapparatus (manufactured by Riken Keiki Co., Ltd., product name: AC-1),the ultra-violet light was irradiated on a thin film in the atmosphereand the number of photoelectrons emitted was measured, so as to make aderivation from the relationship between the incident ultra-violet lightenergy and the number of photoelectrons. The measurement conditions weresuch that the energy range of the incident ultra-violet light was at 3.8to 6.2 eV and the ultra-violet light intensity was at 20 nW.

FIG. 5 is a characteristic diagram illustrating an example of therelationship between the square root of the number of photoelectrons andthe incident ultra-violet light energy. Referring to FIG. 5, the energyat a point of intersection CP2 of a straight line extrapolated to thelow-energy side using a straight-line approximation of a straight-linesegment LN2 at the rise of the characteristic line, and a straight lineextrapolated to the high-energy side using a straight-line approximationof a straight-line segment BG2 at the background is set as the ionizingpotential Ip.

Moreover, the electron affinity Ea is derived as the difference betweenthe ionizing potential Ip derived as described above and the energy gapEg.

Using these methods, the energy gap, the ionizing potential, and theelectric affinity are measured for the individual hole-transportingmaterials, so that combinations of the first and secondelectron-transporting suppressing layers and the light-emitting layerconfiguring the electron-transporting suppressing stack can be selected.

FIG. 6 is a table illustrating the measured values of the energy gap,the ionizing potential, and the electron affinity of theelectron-transporting suppressing layer, the hole-injecting layer, andthe light-emitting layer which configure the organic EL devices ofexamples according to the present invention and of comparative examplesnot in accordance with the present invention.

A First Example

The organic EL device according to the present example is composed froman ITO anode, a hole-injecting layer, an electron-transportingsuppressing stack consisting of three electron-transporting suppressinglayers, a light-emitting layer, and a cathode that are formed on atransparent substrate.

A glass substrate with an ITO electrode is supersonically cleansed withwater, acetone, and isopropylalcohol, and the surface of the anode isirradiated with UV light in the atmosphere for 20 minutes so as toperform a UV ozone process. Then using a vacuum-deposition apparatuswith a vacuum of 1×10⁻⁶ torr and a substrate temperature at 20° C.,2-TNATA and F4-TCNQ as the hole injecting layers are simultaneouslydeposited at the respective deposition rate of 0.5 nm/s and 0.0005 nm/sand are each set to have the thickness of 120 nm. In other words, with2-TNATA being 100 vol. %, the content of F4-TCNQ was set at 0.1 vol. %.

Then, the electron-transporting suppressing stack consisting of threelayers is formed. α-NPD is formed at the deposition rate of 0.1 nm/s andthe thickness of 10 nm, followed by 2-TNATA at the deposition rate of0.1 nm/s and the thickness of 10 nm and then α-NPD at the depositionrate of 0.1 nm/s and the thickness of 10 nm.

As the light-emitting layer, Alq3 is formed at the deposition rate of0.1 nm/s and the thickness of 50 nm. Then on top of Alq3, Al—Li alloy(Li: 0.5 wt. %) is deposited at the deposition rate of 0.02 nm/s and thethickness of 50 nm, forming the organic EL device according to the firstexample. When applying to this device a voltage no less than 6 voltswith ITO as the anode and Al—Li as the cathode, a green-color lightemission was observed.

A First Comparative Example

The organic EL device according to the first comparative example isformed in the same manner as the first example except that 2-TNATA asthe hole-injecting layer is deposited at the deposition rate of 0.5 nm/sand the thickness of 130 nm and α-NPD layer in lieu of theelectron-transporting suppressing stack is formed at the deposition rateof 0.1 nm/s and the thickness of 20 nm.

A Second Comparative Example

The organic EL device according to the second comparative example isformed in the same manner as the first example except that 2-TNATA asthe hole-injecting layer is deposited at the deposition rate of 0.5 nm/sand the thickness of 10 nm, 2-TNATA and F4-TCNQ are simultaneouslydeposited on top of the hole-injecting layer as described above at therespective deposition rates of 0.5 nm/s and 0.0005 nm/s and therespective thicknesses of 120 nm, and α-NPD layer in lieu of theelectron-transporting suppressing stack is formed at the deposition rateof 0.1 nm/s and the thickness of 20 nm.

A Third Comparative Example

The organic EL device according to the third comparative example isformed in the same manner as the first example except that a α-NPD layerin lieu of the electron-transporting suppressing stack is formed at thedeposition rate of 0.1 nm/s and the thickness of 20 nm.

FIG. 7 is a table illustrating the layer configuration of specificelements of the organic EL devices according to the first example, andfirst through third comparative examples. Referring to FIG. 7, it may beunderstood that the light-emitting efficiency of the organic EL deviceof the first example is high relative to the organic EL devices of thefirst to the third comparative examples. It may be understood that theorganic EL device of the first example has not only the light-emittingluminance lower than the second and third comparative examples, but alsothe current density suppressed so that the light-emitting efficiency isenhanced. It may be deduced that the electron-transporting stackconsisting of α-NPD and 2-NATA (with the difference in the electronaffinities of 0.23 eV) reduces the amount of electron current to theelectron-transporting suppressing layer from the light-emitting layer sothat while the light-emitting luminance decreases, the probability ofrebonding with the holes by the electrons enclosed in the light-emittinglayer is increased, enhancing the light-emitting efficiency.

A Second Embodiment

FIG. 8 is an exploded perspective view of the organic EL displayaccording to the second embodiment of the present invention. Referringto FIG. 8, an organic EL display 30 is configured with a glass substrate31, and a cathode 32 formed on the glass substrate in a stripe shape, ananode 34 formed vertically opposing the cathode 31 in a stripe shape,and a stack 33, etc., formed between the cathode 32 and the anode 34.Moreover, the organic EL display 30 is configured with a drive circuit(not illustrated) for driving a voltage to be applied between thecathode and the anode, and an encapsulated container, etc., forpreventing exposure to moisture.

In the organic EL display 30 a voltage is applied to the cathode 32 andthe anode 34 of a desired region causing emitting light in the desiredregion. The feature of the organic EL display is that the organic ELdevice consisting of the anode 34, the stack 33 and the cathode 32 iscomposed from the organic EL device according to the first or secondembodiment as described above. Thus, an organic EL display having a highlight-emitting efficiency and a long service life is implemented.

While preferable embodiments of the present invention have beendescribed in detail, the present invention is not limited to theembodiments so that variations and changes are possible within the scopeof the present invention.

For example, in the present embodiment, the organic EL device may beformed on the substrate by sequentially stacking from the anode side orfrom the cathode side.

1. An organic electroluminescence device, comprising: an anode; acathode formed above said anode; a light-emitting layer containing anorganic light-emitting material that is formed between said anode andthe cathode; a hole-injecting layer on the anode that includes ahole-transporting material and an acceptor; an electron-transportingsuppressing stack between the hole-injecting layer and thelight-emitting layer; wherein the electron-transporting suppressingstack consists of a plurality of carrier-transporting layers, and formsenergy barriers relative to electrons flowing from the light-emittinglayer to the hole-injecting layer.
 2. The organic electroluminescencedevice as claimed in claim 1, wherein said electron-transportingsuppressing stack has arranged a first carrier-transporting layerbordering the anode side of the light-emitting layer and a secondcarrier-transporting layer bordering the anode side of said firstcarrier-transporting layer alternating repeatedly in said sequentialorder, and wherein an electron affinity of the firstcarrier-transporting layer and an electron affinity of the secondcarrier-transporting layer have a relationship in an equation (1)represented asE_(aHT1)>E_(aHT2)   (1) where in the equation (1) E_(aHT1) is theelectron affinity of the first carrier-transporting layer and E_(aHT2)is the electron affinity of the second carrier-transporting layer. 3.The organic electroluminescence device as claimed in claim 2, whereinthe electron affinity of the first carrier-transporting layer and anelectron affinity of the light-emitting layer have a relationship in anequation (2) represented asE_(aHT1)<E_(aEM)   (2) where in the equation (2) E_(aHT1) is theelectron affinity of the first carrier-transporting layer and E_(aEM) isthe electron affinity of the light-emitting layer.
 4. The organicelectroluminescence device as claimed in claim 2, wherein the electronaffinity of the first carrier-transporting layer or the secondcarrier-transporting layer that faces the cathode side of saidhole-injecting layer, and an electron affinity of the hole-injectinglayer have a relationship in an equation (3) represented asE_(aHT)>E_(aHI)   (3) where in the equation (3) E_(aHT) is the electronaffinity of the first or the second carrier-transporting layer thatfaces the cathode side of said hole-injecting layer and E_(aHI) is theelectron affinity of the hole-injecting layer.
 5. The organicelectroluminescence device as claimed in claim 1, further comprising anelectron-transporting layer between said light-emitting layer and thecathode, wherein an ionizing potential of said electron-transportinglayer and an ionizing potential of said light-emitting layer have arelationship in an equation (4) represented asI_(pEM)<I_(pET)   (4) where in the equation (4) I_(pEM) is the ionizingpotential of the light-emitting layer and I_(pET) is the ionizingpotential of the electron-transporting layer.
 6. The organicelectroluminescence device as claimed in claim 2, wherein the firstcarrier-transporting layer is as thick as or thicker than the secondcarrier-transporting layer.
 7. The organic electroluminescence device asclaimed in claim 2, wherein a thickness of the firstcarrier-transporting layer and a thickness of the secondcarrier-transporting layer are in a range of 2 nm to 50 nm.
 8. Theorganic electroluminescence device as claimed in claim 1, wherein acontent of the acceptor in the hole-injecting layer is in a range of0.05 vol. % to 2 vol. %.
 9. The organic electroluminescence device asclaimed in claim 2, wherein either one of said firstcarrier-transporting layer and said second carrier-transporting layer ismade of the same material as the hole-transporting material of saidhole-injecting layer.
 10. The organic electroluminescence device asclaimed in claim 2, wherein said electron-transporting suppressing stackfurther includes a third carrier-transporting layer, and has arrangedthe first carrier-transporting layer, the second carrier-transportinglayer, and the third carrier-transporting layer repeatingsystematically.
 11. The organic electroluminescence device as claimed inclaim 10, wherein said electron-transporting suppressing stack hasarranged the first carrier-transporting layer bordering the anode sideof the light-emitting layer, the second carrier-transporting layerbordering the anode side of said first carrier-transporting layer, thethird carrier-transporting layer bordering the anode side of said secondcarrier-transporting layer, and a further first carrier-transportinglayer bordering the anode side of said second carrier-transportinglayer, and wherein an electron affinity of said firstcarrier-transporting layer and said further first carrier-transportinglayer, an electron affinity of said second carrier-transporting layer,and an electron affinity of said third carrier-transporting layer have arelationship in an equation (5) represented asE_(aHT1)>E_(aHT2)>E_(aHT3)   (5) where in the equation (5) E_(aHT1) isthe electron affinity of said first carrier-transporting layer, and saidfurther first carrier-transporting layer, E_(aHT2) is the electronaffinity of said second carrier-transporting layer, and E_(aHT3) is theelectron affinity of said third carrier-transporting layer.
 12. Theorganic electroluminescence device as claimed in claim 1, furthercomprising a further hole-injecting layer between said anode and thehole-injecting layer, wherein said further hole-injecting layer is madeof a hole-transporting material.
 13. An organic electroluminescencedisplay, comprising the electroluminescence device as claimed in claim1.